I was told about a law that a scribe who is unsure about a letter in a mezuzah/Torah/tefillin, can ask a child who is versed in Aleph-Bet if it is Kosher or not. Where can I find it?
Whether a letter is recognizable to someone who knows how to read letters (but isn't distracted by context clues, which leaves us children who know to read letters but not words) is the most fundamental characteristic of a letter. The vast majority of scribal laws are based on this. (There are a handful of potential exceptions, so don't use this unless you know what you are doing.)
The source for using such a child in a case of doubt is Menachot 29b:
ראמי בר תמרי דהוא חמוה דרמי בר דיקולי איפסיקא ליה כרעא דוי"ו דויהרג בניקבא אתא לקמיה דרבי זירא א"ל זיל אייתי ינוקא דלא חכים ולא טפש אי קרי ליה ויהרג כשר אי לא יהרג הוא ופסול
Rami bar Tamrei, who was the father-in-law of Rami bar Dikkulei, had the leg of the letter vav in the term: “And the Lord slew [vayaharog] all the firstborn” (Exodus 13:15), written in his phylacteries, severed by a perforation. He came before Rabbi Zeira to clarify the halakha. Rabbi Zeira said to him: Go bring a child who is neither wise nor stupid, but of average intelligence; if he reads the term as “And the Lord slew [vayaharog]” then it is fit, as despite the perforation the letter is still seen as a vav. But if not, then it is as though the term were: Will be slain [yehareg], written without the letter vav, and it is unfit. (Translation from Sefaria)